There seem to be a few light ice tools on the market now with B-rated rather than T-rated shafts, such as the Grivel Quantum Tech and the Petzl Aztarex. What's the intended use? When would and wouldn't you trust them?

Even T-rated ice tools mostly have B-rated picks. The strength is really only an issue if you are doing lots of mixed climbing, and mostly for things like torquing. I and a few others here have quantum techs and really like them for waterfall ice.

btw The reason you see B-rated picks on T-rated tools is that often the latter kind don't climb ice as well, because they are thicker. The T-rated picks are usually for more drytooling / mixed climbing where you are more concerned with not breaking a pick vs. ice performance.